Continuously Variable Transmission (CVT) provide a variable speed ratio to maintain an optimal match between the power demand and engine speed, which improves engine efficiency, thereby reducing the fuel consumption and gas emissions.
The most common forms of CVT in use today are V-Belt CVTs. V-Belt CVTs are frictional drives that transmit power through fictional forces. They require large effective radiuses, high belt/pulleys clamping pressure and special transmission fluids in order to provide for sufficient driving torque and to prevent severe wear. During operation, the slippages between the belt and the sheaves under high clamping pressure cause wear and power loss. These characteristics induce to the limitations on torque capacity and transmission efficiency and thus prevent V-belt CVT from being used on vehicles with large displacement engines.
V-Belt CVTs provide a variable speed ratio within a finite range. Additionally, an automobile transmission with V-Belt CVT needs a start-up clutch and a drive- and reverse gearing unit.
To overcome the limitations inherent in the frictional CVTs, attention has been turned to the Positive Engagement, Continuously Variable Transmission (PECVT). To vary the speed ratio, PECVTs vary at least one effective radius. That induces to a meshing problem referred to as the non-integer tooth problem.
Nearly all the published PECVTs, when solving the non-integer tooth problem, become too complex or incompetent or develop another problem known as oscillating output problem. The oscillating output induces to fatigue failure.
U.S. Pat. Nos. 4,235,125, 4,327,604 and 6,053,840 disclose several embodiments comprising two drive shafts and a number of gear-sets including two planetary gear sets (epicyclic gear). The gearsets are coupled together to form a differential system.
The advantage of the embodiments is that they provide a smooth and quiet positive engagement suitable for high torque applications. However, the embodiments do not have any facility to control the speed ratio between the two drive shafts or the torques applied on the shafts. As a result they are unable to control over when and how fast the speed ratio changes.